1. Field of the Invention
This invention relates to a sintered article of silicon carbide exhibiting resistance to oxidation, resistance to thermal shock, resistance to corrosion, and strength at elevated temperatures and, at the same time, possessing high density.
2. Description of the Prior Art
In recent years, sintered articles of silicon carbide, owing to their feature of excelling in resistance to oxidation, resistance to thermal shock, resistance to corrosion, and strength at elevated temperatures, have come to find growing utility in applications to various structural materials, check valves and sealing members designed to handle corrosive liquids, heat-exchanger members for high-temperature furnaces, members expected to withstand heavy friction. Even the desirability of sintered articles which are substantially devoid of pore and are stronger has come to find recognition.
As methods of producing such silicon carbide, (A) chemical vapor deposition (CVD), (B) reaction sintering, and (C) conventional sintering have been known. The method of (A) is capable of producing homogeneous and compact silicon carbide generally only in the form of film and, therefore, is practically, barely suitable for the purpose of coating various materials. The method of (B) which comprises sintering a compact of silicon carbide powder or a mixed powder of silicon dioxide and silicon carbide is capable of producing articles of large dimensions but low density. Therefore, this method is now applied only to production of refractories and heat generators. For the production of sintered articles of large dimensions and high density, the method of (C) is considered as the optimum means.
Incidentally, silicon carbide, which is a compound of high covalent bond property and, therefore, is hard, tough, and stable at elevated temperatures, exhibits very poor sintering property and does not permit easy production of sintered articles when conventional sintering process is applied. Many studies have been being reported concerning adding various sintering aids to improve its sintering property of silicon carbide powder. For example, R. Alliegro et al. Journal of the American Ceramic Society, Vol. 39, pp. 386-389 (1956), the specifications of Japanese Patent Laid-open Publication Nos. 49-007311, 49-099308, 50-078609, 51-065000, 53-067711, and 53-084013 disclose the effect of use of Al, Fe, B, B.sub.4 C, etc. as sintering aids permits production of sintered articles showing low pore contents and high strength.
The strength of sintered articles is effected greatly by the factors of (A) porosity, (B) surface flaw, and (C) grain size. The problem of porosity of (A) can be remarkably solved by using various sintering aids as mentioned above. Although, the sintered articles so produced by the incorporation of such sintering aids, contain extents of microscopic pores. Causing of said (B) surface flaw can be avoided by payment of careful attention to fabrication. The problem caused by the factor from grain size of (C) is most difficult, because of grain growth during the course of sintering, and difficulty of retaining the starting fine grains during the course of sintering. This inevitable growth of grains constitutes itself the cause for the failure of sintered articles to acquire strength beyond a certain limit. This fact is reported by S. Prochazka et al., Am. Caramic Soc. Bull. 52, 885-891 (1973) purporting to conclude that owing to growth of crystal grains, the produced sintered articles fail to acquire any appreciable improvement in strength when using B as a sintering aid.
With a view to eliminating the drawback mentioned above, the inventors have already disclosed that a sintered article of silicon carbide containing erbium oxide and aluminum oxide as an independent composition shows features of high density and extremely fine size of crystal grains in the filed Japanese Patent Application No. 58-190361. Even the sintered article still contains pores measuring approximately 2 .mu.m and is recognized as coarse as 8.5 .mu.m. Thus, the need of producing sintered articles of silicon carbide possessing still higher density and containing crystal grains of still finer size still remains yet to be satisfied.